1. Field of the Invention
The present invention relates to photolithographic equipment for manufacturing semiconductor devices and the like. More particularly, the present invention relates to a scanner of photolithographic equipment, and to a method of scanning a substrate to transfer an image of a pattern to a photosensitive layer on the substrate.
2. Description of the Related Art
Currently, semiconductor devices are being remarkably improved to foster rapid developments in the telecommunications field as well as in information processing equipment such as computers. In such applications, semiconductor devices must operate at high speeds and have the ability to store large amounts of data. As such, the technology of manufacturing semiconductor devices is constantly being improved with an aim toward increasing the integration, reliability, and speed, etc., of the devices.
The manufacturing of semiconductor devices generally includes a deposition process for forming a layer to be patterned on a semiconductor wafer, a photolithography process for forming a mask on the layer to be patterned, an etch process for etching the layer using the mask to thereby pattern the layer, an ion implantation process for implanting dopant ions using the patterned layer as an ion implantation mask, and various heat treatment processes. Among these processes, photolithography is important in attaining the desired critical dimension of the patterned layer. Thus, photolithography is currently the subject of intensive research and development.
Specifically, photolithography is carried out to form and pattern a photosensitive layer, such as a photoresist layer, on the semiconductor wafer. To this end, the photolithography process entails coating the semiconductor wafer with a layer of photoresist, exposing the layer of photoresist, and developing the exposed photoresist. The semiconductor wafer is coated with the photoresist using a spin-coating apparatus and is exposed using an exposure apparatus. The exposure apparatus includes a light source for generating exposure light, such as UV light or X-rays, a reticle bearing an image to be picked up by the exposure light and transferred to the photoresist layer, optics for directing the exposure light through the reticle and projecting the resulting image of the reticle pattern onto the wafer on a reduced scale, and a wafer stage for supporting the wafer and aligning the wafer with the optics so that the image of the reticle pattern is projected onto a desired area of the wafer.
Exposure apparatus of this type includes a stepper in which an image of the reticle pattern is projected onto an entire die of the wafer (unit area of the photoresist layer) such that the die is exposed at once. The wafer stage is then moved (stepped) and the same image is then projected onto another die. The process is likewise repeated until all of the dies are exposed. An alternative to the stepper is a step and scan exposure apparatus (hereinafter, referred to as a “scanner”).
In the scanner, the reticle and wafer are moved parallel to each other in opposite directions. At this time, the exposure light is directed onto the reticle through a rectangular slit. The width of the slit extends in the direction of movement of the reticle during a scan. On the other hand, the length of the slit extends transverse to the direction of movement of the reticle during a scan, and is similar to the diameter of the aperture of the optics of the scanner, e.g., the diameter of the entrance pupil of an objective lens of the optics. As a result, the reticle is illuminated with a rectangular beam of the exposure light. Also, the movement of the reticle causes the reticle and hence, the reticle pattern, to be scanned with the exposure light. Thus, for example, the reticle should be scanned at a speed four times as great as that at which the wafer is moved when the image of the reticle pattern is projected onto the wafer at a reduced scale of about one-fourth.
Furthermore, the width of the slit determines the dose of the exposure light that is incident on the reticle and projects the image of the reticle pattern onto the wafer. Therefore, the masking device of the conventional scanner includes reticle masking blades which are power-driven to produce a rectangular slit having desired dimensions. A conventional scanner including a reticle masking device having such power-driven reticle masking blades is disclosed in U.S. Pat. No. 6,348,303 issued on Feb. 19, 2002, and entitled “Lithographic Projection Apparatus.”
In the scanner disclosed in this document, the reticle masking blades are power-driven by a stepping motor. The stepping motor is operated by a control signal output by a servo control. Therefore, such a conventional scanner produces vibrations, namely, the vibrations of the motor used to move the blades of the reticle masking device. The vibrations can propagate throughout the scanner and, for example, shake the reticle stage. As a result, the light incident on the reticle is not projected onto the wafer in a constant, uniform fashion. Accordingly, the vibrations cause defects to occur in the exposure process, which negatively impacts the yield of the manufacturing process.